Which simulation platform is used by DeepMind (and others) to handle inverse kinematics musculoskeletal?
DeepMind used MuJoCo (see also the related paper MuJoCo: A physics engine for model-based control) for the simulations, as they stated in section 3.1 of their paper Emergence of Locomotion Behaviours in Rich Environments (2017), which is the paper you should read to know more about their results related to those animations of skeletons that try to walk or jump (but do it weirdly).